Halftone photomechanical printing plate and method for producing the same



Aprl 1929- A. H. SPALDING ET AL 1,709,327

HALFTONE PHOTOMECHANICAL PRINTING PLATE AND METHOD FOR PRODUCING THE'SAME Filed Sept. 19. 1925 T f L J 2 -s 7 f f f FG. a e

/2 7 7 I ia. 5 e

91 3( EINEENITORS a 77%), %mm

Ptented Apr; 16, 1929.

UNITED STATES ,1309527 PATENT OFFICE.

mos E. semure, or MANHASSET HILLS, AND JAMES u. CHANKALIAN, or :nw

vom, N. Y., ASSIGNORS ro rowms PHOTO-ENGRAVING COMPANY, A GOBZPOBA- TIONOF NEW YORK.

HALFTONE PHOTOMECHANICAL PBINTING PLATE AN D METHOD FOB PBODUQING THEGAME.

Oontiuatio o! application Serial' No. 583,910. filed August 23, 1922,and serial No. ?33,058, filed August 20, 1924. This application filedSeptember 19, 1925. serial No.- 57,274.

The invcntion relates to a novel process for making half-tone printingplates, and lIl certain aspects thercof it relates' more especially to aprocess for making such plates for use in making stereotyped plates orelectro-type plates.

The distinctive objects and advanta es of the invention will be setforth lereina ter m part, and in part will be ob vious herefrom, or maybe learned by practcng the nvention. i

The invention consists in the novel articles, step or steps, sequence ofsteps and processes herein described.

This application is a contnuaton of our two earlicr and copendingapplications, Ser. No. 583910, filed August 23, 1922 and Ser. No.'733,058, filed August 20, 1924.

The accompanying drawings are dagramnatic in character, as the actualsurfaces and parts are microscopic. They will b e explained and referredto later in the specfication.

The invention is directed to providing a process for making half-toneprintng plates which will have superior printing pro erties, willcorrectly and unifornly pro uce the varying tones of the original, andwhich shall bc especially adapted for making matrices for castingstereotype plates and waX nolds for making electrotype plates.

The process produces plates having the merits or advantages described byreason of the form or structure of the screen figures of the plate, thatis, plates having in the shadows clean, clear, open cells or depressionsof practically the depth between the dots, or other screen figures, inthe lighter and lightcst tones. Such plates also have the screenfigures, including the dots in the lightest tones with sharp pointededges and with smooth downwardly and outwardly sloping sides. There isalso a uniformity of tone throughout a given tone, due to the unifornityof the shape of the dots or equivalent screen figures.

The invention finds one of its widest and most important applications inthe making of zinc half-tone plates for use in making stereotype plates,as in newspaper printing,

and the exemplary manner of carrying out the process is hereinafterdescribed in the frequently but a few minutes are available from thetime a photograph is received until the matrix must be delivered to thestereotype room. Irrespective, however, of the speed of production, thehalf-tone plates must be ,o such high quality and peculiar andcharacteristic properties as'to permit a perfect matrix to be madetherefrom, and then from the matrix stereotype plates which Will giveexcellent results even under the difficult conditions of newspaperrinting.

For certain reasons, among WlllCh are the great speed at which newspaperhalf-tones must be produced, and the relatively great depth required inthe etched half-tone plate it is customary and desirable to give severaetchings or bites to the plate. One of the principal faults in half-toneplates, especially in zine plates, which are etched at great speed andto the required great depth for newspaper stereotyping, is the action ofthe etching fluid in undercutting. This is due to the fact that the acidor etching fluid not only corrodes or eats away the metal vertically butalso laterally. In a properly made half-tone negative for newspaperillustration, all the transparent dots, as well as the areasrepresenting the half-tones are much wider than intended in the finalresult. This is because of the allowance which must be made for thelateral corrosion of the acid or solution used in etching the plate, theaction of the acid as is well known being lateral as well as downward.

The positive photographic prints made from such negatives on the Zincplates, in an acid-resisting material is much stronger than required, tomake allowance for this lateral etching. This lateral etching causes theundercutting and creates shoulders or irregularities in the sides of thedots, or other screen figures, and creates difliculty and damage inwithdrawing the matrices from the finished half-tone plate instereotyping, and the wax mold in electroty ing. The impurities presentin the metal o the plate, and which usually resist the aeid, alsocontribute to the roughness of the screen figures constituting thedesign.

By crcatingthe screen figures in the halftone flatc with true andwell-defined edges for their top surfaces, and smooth downwardly andoutwardly supporting sides, the matrix or the wax mold may be withdrawnfrom the plate without in any wise marring or distorting the impressionmade in the matrix or wax mold by the half-tone late.

By the usual process it is generally ound that one etching of thedeveloped plate does not give the requisite depth, and furthermore whenthe pl-ate is etched twice, While sufi'icient depth is securcd, the dotsor screen figures for the high lights and lighter tones will be formedwith shoulders or heads. This is due to the dragon s blood powder orother resist running partly down the sides of the dots or screen figuresformed by the first etch, the etching fluid then acting only on thebottom parts of the sides of the dots or other screen figures. At thesame time,

the second etch has no efect upon the middle tones and deep Shadows forthe reason that the dragon s blood or other resist has praetic-allyentirely filled the depressions made by the first etch.

F urthermore, the re-etching of the plate tends to produce ragged andirregularly formeddots or other screen figures due to the adherence tothe side wall of the screen figures of the powder of dragon s blood, orother resist, in more or less granulated or disintegrated form andscattered or disposed irregularly -adherent on the side walls of thedots or other screen figures.

In carrying out our process, in the present preferred commercialpractice thereof, a metal plate, such as a zine plate is coated with alight-sensitive coating, and' preferably with -an enanel ,l coating,such as the well-known enanel eonsisting of a solution of glue, white ofcggs and ammonium bichromate. v

After this enamel coating is baked on the plate, it receives the usualexposure through the screen negative. Thereafter the exposed plate isdeveloped, usually with cold water, and thereafter a suitable hardeneris applied to the developed coating, such as a solution of chromie acid,alum, animonium bichromate and water. It is also a common practice tostain the coating with an aniline dye. The foregoing steps arewell-known and are usually praeticed in the art and may be employed inany form or manner best adapted or most suitable as desired.

In the further preparation of the plate by our process, it receivespreferably a preliminary, or repeated preliminary etchings, and then afinal etehing which produces a late having adequate and usually andpreerably equal depth in the depressions between the screen figures inall parts of the plate, and' having screen figures which are true andsymnetrieal in form, and having snooth side walls which are downw-ardlyand outwardly inclined. The screen figures have the surface and the sidewalls thereof very smooth, uniform and true and without the irregularityand raggcdness commonly resulting .from the usual or ordinary processot' plate making.

The first prcliminary etch, applied preferably to a baked enamel plate,when zine is used, may be a dilute nitric acid, say in the proportionsof one part aeid to twenty parts water, the etch being applied usuallyfrom twelve to fifteen seconds, although all of these may be varied asdesircd, so far as concerns the main features of the present invention.

After the etching has been completed, the plate is washed and dried andis then powdered four ways with the dragon s blood or other resist, andis preferably baked after each way of powdering so that the resist willrun down and become attachcd to the sides of the dots to protect themfrom the action of the next etch. v

The next etch is generally similar to the first etch, using the samekind and strength of acid, but the plate is usually etched :from thirtyto forty-five seconds, although all these steps may be varied to suitthe particular case or work. The' plate is then washed, preferably withan alkalinc solut-on, such as a caustie soda solution which willthoroughly clean the plate down to the enamel.

The plate as so prepared, or otherwise properly prepared, is subjectedto the action of a strong or a pure acid, that is, for example, nit-rioacid of full commercial strength, which is applied by means of asuitable brush, such as a glassbrush or one of pig s bristles, the aeidbeing applied by brushing in all directions, and this is usually andpreferably continued until the high light dots reach their extreme pointor become absolutely sharp, and the light tone dots proportionatey so,and the depressions between the dots of the middle tones and in theShadows are formed into clean-walled, open cells equal in depth to thedepressions between the high light dots. The aeid as so applied to theplate has a uniform action throughout, the side walls of the dots, orother figures of the etched plate, being free from adhering powder andthe tendency of the strong aeid is to avoid undercutting. The high lightdots also reach their extreme point, that is, are reduced to a-minimumof area while, if so desired in particular cases, maintaining theirheight in the general level of the plate. The enamel also retains orremains on the surface substantially intact or unimpaired, as conparedto the action llO of the weaker or greatly diluted aeids, as

the water in the latter has a strong solvent efl'ect upon the eamel.

Consdering the invention next from the standpoint of the structure andstructural ste )`s and changes eflected in the plates:

n Fig. 1 a glass screen figure negative 1 is diagranmatically shown,with light-obstructing parts 2 and light-transnitting parts 3.

After this enanel coating is dricd on the plate, it 'cceives the usualexposure to the screen-figure negative. Thereafter the ex-, posed plateis developed, usually with cold water, or with alcohol if it is coldenamel It is also 'a common practice to stain the coating with an anilindye. These stepsare well known and usually practiced in the art, and maybe employed in any'form or manner best adapted or most suitable for theparticular work and kind of plates being made.

In Fig. 2 a metal plate 6 is diagramnatically shown having a designthereon formed by the development of its enanel light-sensitive surfaceafter exposure under the negative 1. The parts' 7 are acid resistantenamel and define the design and effect the tone gradations, and theparts 8 are the parts of the metal bar-ed bythe development in the usualmanner.

As previously stated, it is usually customary or desirable to give theplate a plurality of etchings or btes, usually three or four, althoughthe exact number is not essential. The first etching is for a shortperiod in a Weak acid solution, which may be dilute nitrc acid, say inthe proportion of one part acid to twenty parts water, this etch beingapplied usually from twelve to fifteen seconds, although the mixture andtime may be Varied as found desirable, so far as concerns the mainfeatures of the present invention.

As previously stated, this etch eats out the metal unprotected by thedeveloped enamel surface constituting -the design. It eats not onlydownwardly but also laterally slightly beneath the edges of theprotective enamel coating; The result is that each isolated dot iscorroded or eaten away on the sides, or undercut as it is termed, and issomewhat'rough on its edges( This undercuttng takes place on the edgesof the design, and of all the areas protected on their sui-faces by an.acid resist.

Fig. 3 is a diagram of the plate at this stage, the undercuttings beingshown at 9.

To prevent further undercutting in this same region during the secondetching, and to greatly mininize the total undercutting by thesuccessive ctches or bites, an acid resist, usually a resinous powder ordragon@ blood is applied to the sides of the screen figures so far ascreated by the first etching.

To cause this acid-resisting material to adhere all over the sides ofthe screen figure dots,'the plate is usually powdered our ways, am isleated or baked prefe'ably after every powdering.

When the sides of the screen figures are thus protected (as shown at 10in Fig. 4), the plate is again etched, and this may be done with -thesame etching, fluid, but in present preferred practice the etching iscontinued for a somewhat longer time, say from acted upon by the' firstetching having beenprotected, as stated, from the action of the secondetching solution by the resist 10.

In the case of a third etching, or any number of further etchings, thissame step is repeated. That is, the portion on the sides of the screenfigures WlllCll were affected by the undercutting action of the secondetch are protected by an acid-resisting material in the mannerpreviously described. Thus the portons undercut by the second etch areprotected from the action of the third etch,

and the undercutting action of the third etch is 'thus bothcircunscribed and localized, and oecurs only in a relatively stillsmaller area and both fai-ther 'down and fai-ther out .with respect tothe sides of the screen figure. v

The second powdering is shown in Fig. 6 and the third etching in Fig. 7,and the undercutting thercfrom in Fig. 8 (all diagranatically).

'When sufficient depth between the screen figures, and especiallybetween the high light dots have been effected, the resinous powder orother resistant material, adherent upon and protccting the sides of thedots or other screen figures is removed by a proper solvent, but theenanel coating or equivalent acid resisting material 7 is left adherentupon the tops of the screen figures, that is, upon the printing surfaceof the plate. The acid resist is removed by any suitable agjent analkaline solution, such as a caustic soda solution, being usuallyen'ployed. I

The cleaned platc With the steps or shoulders formcd on the sides of thescreen figures is shown diagraminatically in Fig. 8.

On exan'ination of the etched plate, it will be found thatthe high lightdots are not as fine as they should be and the highest light dots couldbe brought to sharp points nor lUO are the small cavities in the shadowsdeep and wide enough. The principal fault, however, with the etchedplate at this stage is, that on the sides of each dot, line or otherscreen figure, there is a series of steps or shoulders, each of thesebeing slightly undereut. These ragged edges and sides ot' the dots,particularly the undercut side surfaees,.interfere with and prevent thetaking of a. proper papier-neh nold for stereotyping for a wax mold forelectrotyping. Not. only are the screen figures ragged and rough, but'hey tear and deforn the impressinn in the uold as it is withdrawn fromthe plate after the inpression-has been made.

By our invention the rough edges of the etehel screen figures are maderegular and clean-cut, and the side faces of the screen tigures are mades'ooth and outwardly and downwardly sloping, by the elinination of theirregularities due to the undercutting. Also the etching is deepened inall parts of the plate or locally where desired` and the highestligltdots are brought to sharp conical points. The dots in the highest lightsare brought, in some plates, to sharp points, and these points may beVery slightly below the general plane of the plate surface, as shown atthe right in Fig. 9. lVhen the high light dots are very slightly belowthe general surface of the plate, Valuable advantages are realized forsome kinds of work, as -for instance less pressure in the press, longerwear of the plate, and brighter high lights are Secured. The plate isalso better adapted for producing better results in stereotyping. Thereis thus produced a half-tone plate with increased contrast for newspaperprinting, as well as having the other desirable and advantageouscharacteristics set forth. The invention further pro- Yides for doingthis very expeditiously, as is required in certain classcs of work, suchas newspaper work, as already described.

This is done, in the present preferred procedure, by brushing over thewhole plate, which is now protected only by the enamel adherent upon itssurface, with a strong aeid, a pure acid being preferrcd, as forexample, nitric acid of full commercial strength. This is preferablyapplied by means of a suitable brush, such as a glass brush, or one ofpig bristles, the aeid being applied by brushing in all directions.

This will remove quickly the rough steps and shoulders on the sides ofthe dots or other screen figures, will deepen the shallow ravities inthe shadows, will deepen and widn the etching between the lines and dotsof the whole plate and bring the dots in the high lights to still finerpoints, .and the highest light dots'to sharp conical points. Thus allthe screen figures in the half-tone plate have firm clear edges, and thedepressions between the dots of the middle tones and in the shadows areformed into clean-walled, open cells equal in depth to the depressionsbetween the high light dots. One of the valuable features or effects ofthe process is to render coniform all the raised dots in the plate andthe sloping of all the sides of the lines in place of the rough,undercut steps 'and shoulders which were left after the severaletchings.

This vigorous treatment produces an etched plate with deeper, wider andsmoother cavities between the dots, or other screen figures, whichper-mit the forcing of a soft papier-mch fluug down into the deepestportions of the half-tone plate and allows the withdrawal of thematrixwithout injury, and the same is true of the Wax mold, which would beimpossible if the inpressions were taken with all the rough edges andthe undereut steps and shoulders unremoved. It will be understood thatthis treatment may be applied to the entire surface of the plate orloeally, wherever desired, but usually it will be applied over theentire plate and then frequently it will be further applied locally.

From the standpoint of the hysical and chemical ageneies, changes andprocesses involved and occurring, to the best of our present knowledge,the nature and operation of our invention is substantially as follows:

Considering the plate after it has been exposed and developed, and theacid resist .applied thereto, the high light portions of the designconsist of spots or cones of the resist surrounded by the unprotectedmetal 'of the surface of the plate. The shadows of theclesgn appear as acoating of resist 'with tiny holes in it, the bottom of these holesbeing the unprotected -metaL' The various gradations of middle tonesbetween the high lights and deep shadows are variously andproportionately intermediate in form between these two, that is, betweenthe islands of resist with the intervening bare metal of the plate inthe high lights, and the uniform coat of resist with the small holestherethrough in the deep shadows.

Considcring the surface of the plate from a somewhat differentstandpoint, the shadows nay be consider-ed as a coating of resist withcapillary holes therethrough down to the metal surface of the plate,while the high light portions of the plate show very largely unprotectedmetal with a relatively small proportion of the surface thereofprotected by the dots or islands of resist. In these high light areas,the openings through the' resist to the metal surface of the plate areof sufiiciently large dimensione to eliminate the effects that areassociated with or are present with holes of capillary size, such asexist in the shadows. There will be a gradation in the middle tone fromthese free or non-capillary openings of the high lights to away. In thispart 'of the plate it is believed that capillary action plays anegligible part, f present atall, because the distances between thecones or islands of resist are rela- 4 tively great, and this isespecially so when bottoms of these openings.

the coarser half tone screens have been used in making the negative. v v

V In the eep Shadows, on the other extreme,

capillary portions play an important part;

The dilute etching acid enters the holes or capillary openings in theresist and comes into contact with the exposed metal at the tends tohold the dilute acid, which 'is the product of the etching action, inthe hole. There is thus not the free removal of the products of thereaction, as in the case of the etching of the high light areas asalready described, where capillary action does not enter appreciativelyinto ,the process. In the shadow areas, capillarity draws the fresh acidin, and after the etching operation has begun, the action or forcedrawing in the acid is counter-acted by this same capillary attractiontending to hold the spent solution in the hole.

In the variously gradated middle tones, where the openings vary fromslightly larger capillary openings up to the free openings representinga large proportion of the local area, the chemical action is similarlygradated and approximately in corresponding proportions.

As a consequence of these facts, eonditons and physical and chemicalactions, the actual amount of etching is greatest in the high lights,gradating decreasingly through the middle tones to the least amount in'the.

Shadows. This is manifested in the area of etch, or etching action, butis manifested more especially in the depth of the etching ation. Therelative amount of etch per unit surface, for illustration, may be givenarbitrary numbers which would symbolize relative depths of etch. In -agiven case these may be 60, 25 and 15 for h gh lght, middle tone andShadows, respectively.

To change this ratio of etch, it is o nly necessary to change the effectve drvmg capillary force. For instance, f it s desired to decrease theetch in the Shadows, the etching solution should have a surface tensionsuch that it will enter less readly into strong acid is more viscousthan dilute.

Capillary action the capillary. This may be accomplished by using a moreconcentrated acid, which has a surface tension considerably lower thana. more dilute solution of the acid. Having a lower surface tension, ithas less tendeno to go into the capillary, and therefore t etches less.

The high lights, lowever,` are not so affected, since capillarity isnegligible or of Very little importance in the high light areas. Thegradation and action in the middle tones would vary matelyproportionately. strong acid it has been the etching is done in theample, and again using arbitrary numbers for a given case, the ratioofetching in the various areas may become say 85, 10 and 5, from thehigh lights, through the intermediate tones to the Shadows.

It is probable that viscosity acid may also Thus by the use of foundthat most of of the strong play a part in this action, as It would beharder to get the more viscous solution into the smaller hole and,therefore, the etching would be less in the Shadows relatively to thehigh lights.

This etching action of the strong acid is apparent. moreespecially indepth than in extent. In this it is different from the dilute acid etch.This is occasioned probably, to the best of our present knowledge andobservation, by the fact that the gas evolved during the reaction tendsto escape at the juncture ofthe acid and of the metal cones, protectedby the patches of resist, which constitute the high light areas of theplate. The gas must escape in order that the etching may go on.

lt would not push through the liquid between the cones or screenfigures, for it can more easily escape at the contact surface with themetal cones or screen figures. Now, granting a uniform action over thewhole surface, all of the gas from the reaction must pass the surface ofthe metal where the resist lies. Take a point on the plate in the bottombetween two cones or screen figures in the high light, only the gasgenerated by the local etch tends to protect this surface, while thisand all the remainder of the gas generated by surfaces between themiddle point on the side of the cone or screen figure and the topthereof must pass a point at the surface of the metal plate, that is,the top of the cone beneath the layer of resist.

This gas tends to blanket the metal and to prevent the acid coming incontact with it. As this blanketing is least at the middle point betweenthe cones or screen figures, it follows that' the etching action will begreatest at such middle point. Consequently the etching action ismanifested more n depth than in area, as Compared with a dilute acidetch, which is relatively slow correspondingly and approxihigh light.For exnitrogen.

and in which gas evolution' is not such a or factor.

The gases evolved in the case of the strong acid are principally oxidesof ntro'gen mixed with some hydrogen; while in the ease of the diluteacid hydrogen is the principal gas evolved. The top of the conesof metalor screen fi be protected by ocal passivity, due-to the action of theexoessve amounts of oxide of There is no tendency to undercut theresist, for as rapidly as the metal s etched away along the side and topof the cones or screen figures, the strong acid funes, being oxidizing,eat away the resist. With dlute acid, however, -this would remain as ashelf extending over the cone, occasioned by the metal having beendissolved from under the fringe of the resist path or dot.

The' final result of the strong acid eteh, or re-etch is a shifting ofthe relative amount of metal etch toward the high lights, occasioned bya decrease in the force driving the acid into the capillary, whichdecreases ,the relative amount of. etch in the areas where capillarityis important, that is, in the middle tones and in'cre'asingly into theShadows. This ratio is changed, in the arbitrary example heretoforegiven, say from 60, 25 and 15 to 85, 10 and 5.

There is also'a shifting in the extent of the etch, especially in thehigh lights to depth of eteh alone, rather than depth and extentlaterally of the plate. Added to this, there is a uniform oxidation ofthe fringe of resist, thus preventing shelving or socalled undercutting;

This application is `a continuation of our applications Ser. No.583,910, filed Aug. 23, 1922, and Ser. No. '733,058, filed Aug. 20,1924. The praetical shop method has been continuously and very widelyand successfully in commercial use since some little time prior to thefilling of the earlicr application, and the practical use of the processis fully disclosed herein, the actual commercial plates producedpossessing all the commercial excellencies and advantages described.

The physical and Chemical facts, conditions and actions which have beenadded by way of elaboration and explanation, are observed withconsiderable difliculty owing to the microscopic minuteness of structureof the surface of the plate and the rapidity of the various actions,which occur beneath and are largely obsc'ured by the etching fluids.However, the added xplicatory matter represents our best presentknowledge of what actually occurs during the process, from the ph sicaland Chemical standpoint; but it wi l' be understood that the inventionas a valuable and active commercial improveres of the plate may alsoChemical theories and explanations given. It will be understood that thevarious proportions, specific reagents and other details do not limitthe invention; but that departures may be made from the details, withinthe scope of the accompanying claims, Without departing from theprinci-` ples of the invention and without sacrificing itschiefadvantages.

What we claim is:

1. The process of making a half-tone printing plate which comprisessensitizing a plate, exposing the plate to light through a tonedscreen-figure negative, applying a preliminaryetch, applying a resist soas to leave the sides of the screen figuresexposed, and etching theplate to deepen it while producing points with smooth downwardly andoutwardly sloping sides, to efi'cct a desired relation of the varioustones of the half-tone plate.

2. The process of making a half-tone printing plate which comprisessensitizing the surface of a plate, exposing said surface through ahalf-tone screen figure negative, etching the plate and protecting thetops only of the tone-producing screen figures and leaving the sidesthereof entirely unprotected, and i'c-etching the plate, therebyproducing screen figures with smooth downwardly and outwardly slopingsides, to efi'ect a desired relation of the various tones of thehalf-tone plate.

3. The process of making a half-tone printing plate which comprisessensitizing the surface of a-plate, exposing said surface through ahalf-tone screen figure negative, etching the plate and repeatedlyetching with a Weak acid a plate having a surface of tone-producingscreen figures, having a resist thereon and finally etching the platewith a strong acid without a resist, to eii'cct a desired relation ofthe various tones of the half-tone plate.

4. The process of making half-tone printing plate which comprisesapplying a light-sensitive enamel layer to a plate, exposing the plateto light through a half-tone screen figure negative, etching the plate,removing the residue of the resist and etching the plate with only itsresidual enamel as a protection, to eflt'e'ct a desired relation of thevarious tones of the half-tone plate.

5. A half-tone printing plate having aii enamel top surface with a greatnumber of minute screen figures representing the cor-.

in the shadows of uniform tone value with the intervening dcpressionsclean and clear and with smooth sidcs entirely bounding the respectivefigures.

7. The process of producing'a half-tone plate which comprises placing alight-sensitive enamel layer on a plate, exposing the plate to lightthrough a half-tone screen figure negative, and etchingthe plateincluding the steps of localizing the undercutting entirely about theseparated screen figures by repeated etching, and then renov ing theresultant roughnesses from the sides of the screen figures of the plate,to cfi'cct a, desired relation of the various tones of the half-toneplate'.

8. The process of producing a half-tone plate which comprises placing alight-sensitive enamel layer on a plate, exposing the plate to lightthrough a half-tone screen figure negative, and etching the plateincluding the steps of localizing the undercutting entirely about theseparated screen figures by repeated etching, and then removing theresultant roughnesses from the sides of the screen figures of the platebythe application of a strong aeid, t etect a desired relation of thevarious tones of the half-tone plate.

9. The process of producing a half-tone plate which comprises placing alight-sensitive enamel layer on a plate, exposing the plate to lightthrough a half-tone screen figure negative, and etching the plate by aplurality of biles and proteoting the undercutting entirely about theseparated screen figures made by one bite from the action of asucceeding bite, and; removing the resultant roughnesses from the sidesof the screen figures of the plate, to efi'ect a desired relation of thevarious tones of the half-tone plate.

10. The process of producing a half tone plate which comprises etchingthe plate by a plurality of bites and protecting the undercutting madeby one bite from the action of a succeeding bite, and removing theresultant roughnesses from the sides of the screen figures of the plateby scrubbing the plate with strong acid.

11. The process of producing a halftone plate which comprises creatingan enamel design on the surface of the plate, etching the plate by aplurality of bites, and protecting the undercutting made by one bitefrom the action of a succeeding bite, and removing the resultantroughnesses from the sides of the screen figures of the plate byscrubbing the surface of the plate with acid with all resist removedexcept the surface enamel.

12. A half tone printing plate having the high light dots with smoothconical sides terminating in points very slightly below the plane of thesurface of the plate.

13. The process. of producing a half-tone -printing plate whichcomprises sensitizing the plate with an enamel layer, exposing the plateto light through a half-tone screen' figure negative, repeatedly etchingthe plate entirely about the separ'ated screen figures, with solutionshaving varying surface tensions to regulate the relative depth andlateral extent of the different etches, to eflt'ect a` desired relationof the various tones of the half-tone plate.

14. The process of producing a half-tone printing plate which comprisessensitizing the plate with an enamel layer, exposing the plate to lightthrough a half-tone screen figure negative, repeatedly etching the plateentirely about the separated screen figures, with solutions of differingdegrees of viscosity to regulate the relative depth and lateral extentof the different etches, to efl'ect a desired relation of the varioustones of the half-tone plate.

15. The 'process of producing a half-tone print-ing plate whichcomprises sensitizing the plate with an enamel layer, exposing the plateto light through a half-tone screen figure negative, repeatedly etchingthe plate entirely about the scparated screen figures, with solutions,and varying the character of a succeeding etching solution to securedepth of etch with practically no lateral etch.

16. The process of producing a half-tone printing plate which comprisesapplying a light-sensitive layer to the surface of the plate, exposingand developing the plate to create screen figures thereon, preliminarilyetching the plate with dilute acid, and subse quently etching the platewith strong acid without a resist being applied preliminary to saidetching.

17. The process of producing a halt-tone printing plate which comprisesapplying an enamel light-sensitive layer to the surface of the plate,exposing and developing the plate, to create screen figures thereon,baking the developed enamel coating, preliminar'ly etching the platewith dilute acid, and subsequently etching the plate with substantiallyundiluted aeid without a resist being a plied preliminary to saidetching.

18. T e process of producing a half-tone printing plate which comprisesapplying a light-sensitive layer to thesurface of the plate, exposingand developing the plate, to create screen figures thereon,preliminarily etching the plate with dilute acid, applying a resist tothe surface of the plate, etching the plate with a dilute acid, andsubsequently etching the plate with strong acid without a resist beingapplied prelininary to said etching.

19. The process of producing a half-tone printing plate which comprisesapplying an enamel light-sensitive layer to the surface of the plate,exposing and developing the plate, to create screen figures thereon,baking the developed enamel coating, prelimnarily etching the plate withdilute acid, applying :L rcsist to the surface of the plate, etching theplate with a dilutc acid, and sul.)- sequcnt-ly etching the ulate withstron aeid without a resist being applierl prelin-unary to said etching.

20. A halt-tone printing plate having its screen figures in the highlights uniformly reduced to points and in the Shadows of uniforn tonevalue with the intervening depressions clean and clear and with smoothsides.

21. The process of proclucing a photomechanieal printing plate whichcomprises applying a light-sensitive layer to the surface of the plate,exposing the platc through a halt-tone screen-figure negative and (le-Veloping the plate, prelininarily etching the plate with (lilute acid,and subscquently directly applying a strong etching acid hy frictionalmeans applied on the surface of the plate.

22. The process of 1'roducing a photonechanical printing plate -whicheomprises applying a light-sensitive layer to the surface of the plate,cxposing the plate through a half-tone screen-figure negative and(leveloping the plate, prelininarily etehing the plate with dilute acid,and suhsequently directly applying a strong etching aeid (lirectly toall parts of the design except the faces of the screen figures.

23. The process of producing a photomechanical printing plate whichcomprises applying a light-sensitive layer to the surface ot the plate,cxposing the plate through a halt-tone screen-figure negative andcleveloping the plate, applying an acid resist to the developed surfaceof the plate, pre-.

lininarily etching the platc with dilute acid, and subsequently directlyapplying a strong etching acid to the plate.

24. The process of producing a photomeehanical printing plate whichconprises applying a light-sensitive layer to thesurface of the plate,exposing the plate through a halt-tone screen-figure negative and de-Veloping the plate, applyin an acid resist to the developed surface otthe plate, preliminarily etching the plate with dilute acid, andsubsequently directly applying a strong etching acid by frictional meansapplied on the surface of the plate.

25. The process of producing a 'photomechanical printing plate whicheomprises applying a light-sensitive layer to the surface of the platc,exposing the plate through a halt-tone screen-figure negative and. de-Veloping the plate, applying the acid resist to the developed surface ofthe plote, prclininarily etching the plate with dilute acid, applying anacid resist to the etched surface, etehing the plate with a dilutc acid,and subsequently directly applying a strong etching acid to the entiresurface of the design on the plate except the faces of the screenfigures.

26. The process of producing a photoncchanical printing plate whichcomprises applying a light-sensitive layer to the surface of the plate,exposing the plate througia half-tone screen-figure negative anddeveloping plate, prelininarily etching the pinte with dilute acid, andsubsequently directly applying a strong etchiug acid to the plale Withthe sidcs ot' the screen figures unprotected by any rcsist.

27. The process of producing a halt-tone printing plate which comprisesapplying an enanel to the surface ot' the plale to create screen figuresthercon,exposing and dcvcloping the plate, prelininarily etching theplate with dilute acid, removing all resist except the enanel, andsubsequently etching the plate with strong acid without a resist beingapplied prcliminary to said etching.

28. Ihe process of produeing a half-tone printing plate which comprisesapplying a light-sensitive enanel coating to the pl-ate, exposing theplate through a halt-tone s'l'ctll figure negative, applying a resist to'hc platc, and etching and cleaning the plate with a strong aeid toinsure the renoval of all resistaut matter except the cnanel surface otthe plate.

29. The process of producing a half-tone plac to he used alone inprinting, which conprises the steps of exposing a lightsensitive plotethrough a halt-tone screen figure negative to truly reproduce the lighttones ot the original, repeatedly etching the plate, localizing theuudercutting by repeated etching, and then removing the resultantroughnesses from the sides ot' the screenrfigures of the plate.

30. The process of producing a half-tone plate to be used alone inprinting which conprises the steps of exposing a lightsensitivc platcthrough a half-tone screen figure negative to truly rcproduce the lighttones of the original, repeatedly etching the plate, localizing theundercutting hy rcpeated etching, and then removing the resultantroughnesses froni the sides ot the screen figures of the plate by theapplication ,of a strong acid.

31. The process of producing a halt-tone plate to be used alone inprinting, which eonprises the steps of cxposing a lightsensitive platethrough a half-tone screen figure negative to truly rcproduce the lighttones of the original, repeatedly etching the plate, localizing theundercutting by repeated etching, and then removing the resultantroughnesses from the sides of the screen figures of 'the plate byscrubbing'the plate With aeid.

32. The process of producing a half-tone plate to be used alone inprinting, which llO comprises exposing a light-sensitive plate through ahalf-tone screen figure negative to truly reproduce the light tones of'the original, repeatedly etching the late, etching the plate by aplurality of bites and protecting the undercutting made by one bite fromthe action of a succeeding bite, and removing the resultant roughnessesfrom the sides of the screen figures of the plate by the application ofstrong acid.

33. The process of producing a half-tone plate to be used alone inprinting, which comprises exposing a light-sensitive plate throughahalf-tone screen figure negative and creating an enamel design on thesurface of the plate, etching the plate by a plurality of bites, and-protecting the undercutting made by one bite from the action of asucceeding bite, and removing the resultant roughnesses from the sidesof the screen figures of the plate by scrubbing the surface removedexcept the surface enamel.

34. The process of producing a half-tone -plate to be used alone inprinting, which comprises exposing a light-sensitive plate through ahalf-tone screen figure negative and creating an enamel design on thesurface of a plate, etching the plate, deepening the plate andsharpening and smoothing the screen figures ofthe plate by scrubbing itwith acid with the plate unprotected except by the enamel of the surfacedesign.

35. A half-tone printing plate to be used alone in printing, having thehigh light dots with smooth sides terminating in points very slightlybelow the plane of the surface of the plate.

In testimony whereof, we have signed our names to this specification.

AMOS H. SPALDING. JAMES M.- CHANKALIAN.

